Elimination of NOx from the exhaust of a lean-burn engine is a challenging problem because it can only be removed via reduction. For example, a diesel engine operating under lean conditions with excess oxygen provides excellent fuel economy, however the reduction of NOx in these oxidizing conditions is very difficult. For a gasoline engine operating under stoichiometric conditions three-way catalyst technology can be used to reduce HC (hydrocarbon), CO and NOx emissions. However, as with the diesel engine, the three-way catalyst is not able to reduce NOx under lean conditions. Therefore, with respect to both engine types there remains a need for the development of new catalyst systems to remove NOx from the exhaust gas under lean-burn conditions.
The conversion of NOx to stable nitrates, and storing the nitrates under lean conditions, followed by reducing the nitrates or “stored NOx” to dinitrogen under rich conditions has been investigated with some success. Some of the early NOx storage catalysts fitted to the exhaust systems of vehicles contained barium compounds. These catalyst systems stored NOx when the engines operated under lean conditions, and reduced the stored NOx when the gas was made rich. See, B. J. Cooper, et al. in Catalysis and Automotive Pollution Control, eds. A. Curcq and A. Frennet (Elsevier, Amsterdam, 1987) p. 117. Unfortunately, the present levels of sulfur compounds in fuel and the resulting SO2 in the exhaust gas has prevented the commercial introduction of these catalysts, which are relatively susceptible to sulfur poisoning.
A new generation of NOx storage catalysts tested against simulated exhaust gas from a lean-burn gasoline engine has provided an average NOx conversion of more than 90% over a test cycle in which the air/fuel ratio was cycled between lean and rich. See, W. Bögner, et al., Applied Catalysis B7 (1995) 153. The feed gas was alternated between oxidizing (lean) and a short reducing (rich) period every two minutes or so resulting in NOx storage during the lean period and conversion to dinitrogen during the rich period.
Over the past decade significant efforts have been made toward the development of NOx storage catalysts containing platinum as well as other noble metals as an oxidative and reductive catalyst. Barium has been used as the primary NOx storage material though other NOx storage components have also been investigated. The use of transition metals to improve resistance to sulfur poisoning has also been investigated. See, K. Yamazaki, et al., Applied Catalysis, B 30 (2001) 459.
Lean burn gasoline and diesel engines not only improve the fuel efficiency of automobiles, but also lead to a reduction in the emission of greenhouse gases. Impeding the widespread implementation of lean burn engines is the inability of current three-way catalytic converters to reduce nitrogen oxides under oxidizing lean conditions. Extensive research has been performed in search of alternative catalysts that will reduce NOx in oxygen rich environments under steady state conditions, but an acceptable catalyst has not yet been discovered.